1. Field of the Invention
This invention relates to an apparatus and method for joining together sheet materials, and more particularly to a hemming apparatus and method for joining together two superimposed sheet metal layers.
2. Description of the Related Art
Heat shields and other articles having two superimposed sheet metal layers are typically joined together in a conventional hemming operation, wherein the outer periphery of one of the layers is crimped over the outer periphery of the other layer. Depending on the complexity of the article, the hemming or crimping operation can be time consuming and labor intensive. For example, the shape of an upper heat shield located between the floor and catalytic converter of motor vehicles is quite complex. These types of heat shields typically have outer peripheries with complex curvatures that may extend along three mutually perpendicular axes and/or combinations thereof. During the hemming operation, the heat shield must be positioned and repositioned as many as six times or more indifferent presses in order to properly crimp one layer over the other while following the complex curvatures. Each press typically has a set of dies that conforms to a portion of the outer peripheral shape of the upper and lower sheet metal layers for a particular orientation of the heat shield. When the shape of the heat shield is modified, new sets of dies must be manufactured. The dies are costly to manufacture and difficult to maintain, and thus contribute to the overall cost of the heat shields.